1. Field of the Invention
This invention relates to expansive cement compositions.
2. Summary of the Prior Art
Compositions which contain hydratable cements (generally Portland cement) tend to shrink on drying. It is known to counteract this undesirable tendency by including in the composition an expansion additive which expands at the correct stage during the drying of the composition [see for example Gen. Civ. 109, 285 (1936, H. Lossier), U.C. SESM Report No. 72-13 (1973, G. Komendant et al.), Plan. Bau 2, 351 (1951, H. Bickenbach), Concrete Technology and Practice, 3rd Edn, 359 (1969, W. H. Taylor) and U.S. Pat. Nos. 3,519,449, 3,649,317, 3,801,339, 3,883,361, 3,884,710, 3,947,288 and 4,002,483]. In many of the known expansion additives, the active ingredient is free lime (CaO), also known as quick lime, which expands by about 100% in volume when hydrated, the other ingredients being present to prevent the hydration taking place until the desired stage in the drying of the cement composition. Free lime is one of the ingredients of Portland cement, but an excess of it is normally regarded as undesirable, and the addition of free lime to cement compositions is avoided. Free lime does not serve as an expansion additive because it is very rapidly hydrated and therefore expands far too early in the setting process to be effective. It is also known (see for example U.S. Pat. No. 3,106,453 and German Pat. No. 1,216,753) to subject free lime to certain specific treatments with water and/or CO.sub.2 to produce a lime which hydrates at a slower rate, but the products obtained are ineffective as expansion additives. Likewise the products obtained by exposing lime to the atmosphere, which products contain Ca(OH).sub.2 and CaCO.sub.3, are ineffective as expansion additives. It has also been proposed in U.S. Pat. No. 1,732,409 to make quick-setting lime products which contain a high proportion of lime, at least some of the lime being in the form of a product obtained by heating finely divided lime in an atmosphere containing CO.sub.2 at a temperature of 500.degree. to 850.degree. C. until the increase in weight is 3 to 40% based on the weight of the starting material.